This invention is in the field of nuclear magnetic resonance (NMR) apparatus and relates in particular to multiple tuned birdcage coils, as well as to methods of producing such coils.
It has been known to structure a radio-frequency coil in a birdcage form in order to obtain a highly homogeneous magnetic field as described, for example, in U.S. Pat. No. 4,694,255 issued Sep. 15, 1987 to C. Hayes and xe2x80x9cExperimental Design and Fabrication of Birdcage Resonators for Magnetic Resonance Imagingxe2x80x9d (T. Vullo, et al., Magnetic Resonance in Medicine, 24, 243 (1992)). Birdcage coils are so called because of their general structure having a pair of loop-shaped conductive elements (the xe2x80x9cringsxe2x80x9d) separated in a longitudinal direction and a plurality of conductive segments (the xe2x80x9clegsxe2x80x9d) evenly spaced about the circumference of and interconnecting these two loop-shaped conductive elements. Capacitors are inserted either in the legs for a low-pass coil, or in the rings for a high-pass coil. It has been recognized that a birdcage coil with a large number of legs, or a millipede coil, has increased field homogeneity and that a spiral version of the standard birdcage coil with conductor legs twisted around the circumference instead of extending axially straight demonstrates improved homogeneity. Throughout herein the term xe2x80x9cbirdcage coilxe2x80x9d will be used for birdcage coils of both the so-called xe2x80x9cstraight kindxe2x80x9d with straight legs extending parallel to the central axis and the so-called xe2x80x9cspiral kindxe2x80x9d with helically spiraling legs. The B1 field generated by such a birdcage coil of a spiral kind rotates around the central axis. The angle by which the B1 field rotates between the two rings will be hereinafter referred to as the xe2x80x9ctwist anglexe2x80x9d. The twist angle for a birdcage coil of the straight kind is zero. The twist angle of a birdcage coil of a spiral kind may be either positive or negative, depending on the direction of the spiraling of the legs.
Birdcage coils are generally tuned at a discrete frequency. When two resonance frequencies are required, one of basic approaches would be by way of double tuning a single coil with lumped elements, while another approach would be by single tuning two coils and putting them together. When two coils are put together, say, coaxially one inside the other, however, there arises the problem of inductive coupling between them. It has therefore been an important problem to minimize the interaction between closely arranged coils, or to make them mutually transparent.
It is therefore an object of this invention to provide a multiple tuned birdcage coil structure having a two or more single-tuned birdcage coils arranged coaxially one inside another with minimized mutual interaction among them.
It is another object of this invention to provide a method of producing such a multiple tuned birdcage coil structure.
It is still another object of this invention to provide methods of producing single tuned birdcage coils and double tuned pairs of such birdcage coils with minimized mutual interaction between them, say, as a step in producing a multiple tuned birdcage coil structures with three or more birdcage coils.
Each of the single-tuned birdcage coils, or millipede coils, to be arranged coaxially one inside another to form a multiple tuned birdcage coil structure embodying this invention may be characterized as having birdcage legs which helically twist around such that the direction of its B1 field rotates by a specified angle (defined above as the twist angle). The twist angles of any two of the assembled individual birdcage coils are different by an integral multiple of 360 degrees. As a specific example, a double tuned birdcage coil structure may be formed by assembling a birdcage coil with straight legs (with twist angle of zero degree) and another with spiraling legs with a twist angle of 360 degrees. Alternatively, two coils with spiraling legs, one with a twist angle of +180 degrees and the other with a twist angle of xe2x88x92180 degrees may be assembled.
Aside from this relationship among the twist angles (which may be zero or non-zero) of the birdcage coils to be assembled, the invention does not impose many requirements as to their types. They may each be of a type with the birdcage legs arranged in an interdigital configuration, of another type with legs protruding from each of the rings towards the other in a radially overlapping positional relationship with respect to the central axis, or of still another type with each of the legs connected to one of the rings at one end and capacitively coupled to the other at the other end. Although a prior art birdcage coil usually has two metallic rings separated along the central axis, two of the birdcage coils which are assembled according to this invention may share a common ring, or a common ring may be capacitively coupled to the legs of two birdcage coils.
Such a multiple tuned birdcage coil structure may be produced by providing a plurality of single tuned birdcage coils with twist angles related as required above and inserting one inside another coaxially. Alternatively, a double tuned birdcage coil structure with only two birdcage coils assembled coaxially one inside the other may be produced as a first step. This may be done by providing a so-called printed circuit substrate with metallic sheets on both surfaces of an insulating sheet, etching the metallic sheets in specified patterns, and then rolling it into a cylindrical form. Each pattern includes a set of plurality of mutually separated and parallel linearly elongated members, and the sets of these elongated members are oriented obliquely to each other such that, as the substrate is rolled, they turn into the legs of two birdcage coils. The patterns are designed such that each of these two coils thus formed has a specified twist angle and that the difference between their twist angles will be an integral multiple of 360 degrees.